Bones not rotating properly in gimbal mode
That red line that shows up when you are rotating means you are locking the rotation to the X axis by pressing X - this will be the world's X axis.

When you first start to rotate by only pressing R the rotation is restricted to the view orientation, meaning it will appear to rotate in a circle as seen in the view you are using.

You can also lock the transform to the objects local X axis by pressing XX.

This can also be adjusted to the Y or Z axis when moving, rotating and scaling. Also to remove an axis from the transform also press Shift. eg pressing S then ShiftX will scale on both Y and Z axes.

You can change the transform orientation used by the widgets in the header menu.



Sometimes you can also find it helpful to use RR which puts you in trackball rotation - that is not locked to any particular axis.